Radiation-Induced Defects in Uranyl Trinitrate Solids

Actinides are inherently radioactive; thus, ionizing radiation is emitted by these elements can have profound effects on its surrounding chemical environment through the formation of free radical species. While previous work has noted that the presence of free radicals in the system impacts the redox state of the actinides, there is little atomistic understanding of how these metal cations interact with free radicals. Herein, we explore the effects of radiation (UV and γ) on three U(VI) trinitrate complexes, M[UO₂(NO₃)₃] (where M=K⁺, Rb⁺, Cs⁺), and their respective nitrate salts in the solid state via electron paramagnetic resonance (EPR) and Raman spectroscopy paired with Density Functional Theory (DFT) methods. We find that the alkali salts form nitrate radicals under UV and γ irradiation, but also note the presence of additional degradation products. M[UO₂(NO₃)₃] solids also form nitrate radicals and additional DFT calculations indicate the species corresponds to a change from the bidentate bound nitrate anion into a monodentate NO₃ ^• radical. Computational studies also highlight the need to include the second sphere coordination environment around the [UO₂(NO₃)₃]^0,1 species to gain agreement between the experimental and predicted EPR signatures.